QuestionThe check engine light has recently come on and I've noticed a severe
decrease in fuel efficiency. I had an Advance Auto Parts and VIP shop both
diagnose it. I was not able to get to someone who "knows" Subarus, but they
both said that the knock sensor and the O2 sensor were bad; more
specifically the 1st O2 sensor. I hesitate to replace these as I don't trust the
accuracy of either places diagnosis. I understand that there could be a
correlation between poor fuel mileage (18 mpg) and the malfunctioning O2
sensor.
Could it instead be a timing issue? I only ask this because the car is lacking
power, and chugs at times, especially at 60 mph in 5th gear. Any advice you
may have would be greatly appreciated.
-Tyler
AnswerHey Tyler,
We see a fair amount of problems with both the O2 sensor and the Knock sensor on that vehicle and it wouldn't surprise me if one or both were giving you some trouble. But having said that it is truly hard to know how best to advise you. Its nothing against either Advanced Auto parts or VIP, but truly diagnosing a problem with a car can be a challenge. There are two types of OBDII codes conditional and component. Component codes are pretty straight forward to investigate and can be done with a scan tool and a volt-ohm meter in some cases. Where you can run into trouble is if you are dealing with a conditional code such as O2 rich or O2 lean, a conditional code really requires a thorough analysis and this isn't usually a good task for a parts store. The timing on your Subaru is controlled by the computer using many data points including the frequency omitted by the knock sensor. Another thing to look at is the coil towers and plug wires for signs of corrosion or burning. Which code was set by the way?
Hope this helps point you in the right direction
Justin
